A piston ring used in an engine of an automobile is mounted in a ring groove formed on an outer peripheral surface of a piston. The piston ring has an outer peripheral surface to slide on an inner peripheral surface of a cylinder bore and one side face to contact a side wall of the ring groove, thereby restraining a fuel gas from leaking from a combustion chamber side to a crank chamber side, that is, restraining a blow-by gas. Restraining the blow-by gas enables prevention of the formation of sludge caused by contamination of a lubricating oil that may lead to adherence of an oil ring and degradation in performance of the engine.
The piston ring, to facilitate mounting thereof in the ring groove, is formed in a split ring shape with a piston ring gap. Thus, restraining the blow-by gas from passing through the piston ring gap should also be a concern. To restrain the blow-by gas from passing through the piston ring gap, it is important to minimize the piston ring gap within a range in which piston ring end faces do not contact each other due to thermal expansion caused by passage of a combustion gas.
Further, the piston rings having the piston ring gap are commonly provided with chamfered edges on the outer peripheral edges of a pair of piston ring end faces facing each other across the piston ring gap as illustrated in, for example, the PLT1 set forth below. In this case, since a chamfered amount severely affects a passage area of the blow-by gas in the piston ring gap, the chamfered amount is minimized so as to repress the occurrence of the blow-by gas.